1. Field of the Invention
The present invention relates to a creasing device and to an image forming system.
2. Description of the Related Art
What is called saddle-stitched or center-folded booklet production has been conventionally performed. The saddle-stitched booklet production is performed by saddle stitching a sheet batch, in which a plurality of sheets delivered from an image forming apparatus is bundled together, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause outer sheets of the sheet batch to be stretched at a folded portion by an amount greater than inner sheets. Image portions at the folded portion on outer sheets can thus be stretched, thereby causing damage, such as come off of toner, to the image portions in some cases. A similar phenomenon can occur when other folding treatment, such as z-fold or tri-fold, is performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
Creasing (scoring) devices that, to prevent come off of toner, creases at folded portion of a sheet batch prior to a folding treatment where the sheet batch undergoes single fold or the like so that even outer sheets is liable to be folded have already been known. Known examples of devices of this type include a device disclosed in Japanese Patent Application Laid-open No. S60-262771.
This known example device includes a conveying belt that conveys sheets, a pressing member that uplift a sheet-conveying surface of the conveying belt, and a V-belt that rotates in pressure contact with a sheet on the conveying belt uplifted by the pressing member and forms a crease (fold stripe) in advance in sheets, which are to be saddle stitched, for quality enhancement of saddle stitching.
Known creasing devices are configured such that a sheet detection reference position for determining a crease position, and a sheet detection reference position for a unit that performs folding treatment in a subsequent process differ from each other. More specifically, for instance, a sheet is creased at a predetermined position with reference to a front edge of a sheet, the sheet is folded at a predetermined position with reference to a rear edge of the sheet in a folding treatment of a subsequent process. Meanwhile, even sheets of a same size can be dimensionally varied because of allowance or the like. Accordingly, such a configuration as discussed above can cause an offset between the crease position and the fold position, resulting in degradation of folding quality.
The known technique discussed above allows a crease to be formed in sheets, which are to be saddle stitched, on a fold stripe in advance; however, the technique gives no consideration to the offset, as discussed above, where a sheet detection reference position for determining a crease position, and a sheet detection reference position for a unit that performs folding in a subsequent process differ from each other.